Lack of adequate physical activity has numerous negative health consequences, but the majority of U.S. adults fail to attain even minimal recommended levels of daily physical activity. Most programmatic interventions focus on increasing recreational physical activity, but have modest initial and even weaker long-term effects, enroll only the motivated few, and subsequently have limited public health impact. An alternative approach is changing environments, infrastructure, or policies in ways that affect daily physical activity. One promising approach, although not designed specifically to target physical activity, are attempts to increase transit use (e.g., bus, light rail). Cross-sectional findings suggest that transit users accumulate significant amounts of physical activity by walking to/from transit access points (e.g., bus stops) and have lower chronic disease risk. Unfortunately, transit use in the U.S. is low, but there is growing interest in increasing transit se to reduce fossil fuel energy consumption and congestion. Bus rapid transit (BRT) is becoming an increasingly popular strategy to try to increase transit use in the U.S. and elsewhere. This alternative to regular bus service reduces commute times, improves service reliability, and is also markedly cheaper to initiate and implement than light rail. In spring 2014, King County Metro Transit will begin BRT service on the final two lines of its BRT system. This study takes advantage of this natural experiment and aims to examine the change in physical activity, specifically transit-related walking, in a prospective cohort design involving individuals living close to BRT (n=342) versus those living farther away from BRT (n=342) but who are otherwise similar demographically and in initial built environment. Before, soon after, and >2 years after BRT begins, participants will wear accelerometer and global positioning systems (GPS) devices for 7 days and record travel behavior. These data are integrated provide reliable and valid measures of physical activity, walking, and travel behavior. This study will also examine potential mediators of the relationship between BRT exposure and physical activity. Attitudes about travel (to be measured by survey) have the potential to change from before to after BRT as In Motion, a neighborhood-based social-marketing and incentive program that encourages residents to decrease drive- alone travel, will be implemented by the local transit agency along these new BRT lines. In addition, the change in built environment near and at BRT stations, particularly environmental factors known to influence walking, will be investigated as potential mediators. Finally, this study seeks to obtain critical cost effectiveness information needed by decision makers, inputting the costs of BRT implementation (relative to regular bus service) versus the likely benefits of increased physical activity and lower injury. Findings from this timey health-oriented evaluation of BRT will inform decision makers about the potential for infrastructure changes to impact the critical health behavior of physical activity, as well as the relative costs and health-related benefits of such changes. PUBLIC HEALTH: Inadequate physical activity is common among U.S. adults and is a significant contributor to chronic disease. It is critical to evaluate the impact of major transportation infrastructure changes on physical activity and their cost effectiveness from a health perspective. Such evaluation will guide future strategies to improve public health.